1. Field of the Invention
The present invention relates to a method for forming a resist pattern, a method for producing a circuit board, and a circuit board. More specifically, the present invention relates to a method for forming a landless or small-land-width resist pattern suitable for realizing a high-density circuit board and a fine wiring pattern on a circuit board, a method for producing a circuit board, and a circuit board.
2. Description of the Related Art
As electronic equipment has been reduced in size and provided with multiple functions in recent years, a high-density circuit board and a fine wiring pattern on a circuit board have also been realized. A means for achieving such conditions is a multilayer circuit board. As shown in FIG. 45, in a circuit board formed by stacking multiple wiring layers, the layers are conducted to each other through a pore such as a through-hole or a non-through-hole (hereinafter collectively called “hole”) having an inner wall covered with a conductive layer or being filled with a conductive layer, which is generally called a through-hole 31, a via hole 32 or an interstitial via hole 33.
FIG. 46 is a schematic view of a through-hole from above. A conductive layer called a land 18 is formed around a through-hole 3. There are various types of lands such as square, round, oval and deformed lands. A round land is often used in terms of occupied area and easiness in designing. A landless or small-land-width through-hole is needed for a high-density circuit board. Here, the land width (Lw) means a minimum width of an annular conductor around a through-hole in the case of a round land. When the diameter of a through-hole when the hole is formed is D0 and the diameter of an annular conductor of a round land is D, the term “landless” means a land width Lw=(D−D0)/2 of 0 and the term “small-land-width” means a land width Lw=(D−D0)/2 of more than 0 and 40 μm or less.
A subtractive method, an additive method and a semi-additive method are known as a method for producing a circuit board. The additive method is a method of providing a plating resist layer in a non-circuit part on the surface of an insulating substrate and forming a conductive layer in a part corresponding to a circuit part by electroless plating or the like. The method is advantageous for forming a fine circuit; however, since all conductive layers are formed by electroless plating, the production cost is high, disadvantageously.
The semi-additive method is a method of providing a plating resist layer in a non-circuit part of an insulating substrate having a thin conductive layer on the surface, forming a conductive layer in a part corresponding to a circuit part by electrolytic plating, removing the plating resist layer in the non-circuit part, and then removing the thin conductive layer in the non-circuit part by flash etching to form a circuit. Since electrolytic plating enabling high-speed operation can be used, the method can be advantageously used as a method for forming a fine circuit. The production cost is lower than that of the additive method. However, it is difficult to form all wiring patterns with a uniform thickness by electrolytic plating, and quality management is difficult, disadvantageously.
The subtractive method is a method of providing an etching resist layer in a circuit part of an insulating layer provided with a conductive layer on the surface, and removing the uncovered conductive layer in a non-circuit part by etching to form a circuit. Since an image is weakened by side etching of a conductive layer, for example, the method has limitations as compared with the other two methods in terms of formation of a fine circuit. However, the method can prepare a circuit board by simple treatment, has high productivity, has the lowest production cost, and is most widely used.
An etching resist layer and a plating resist layer are formed by screen printing, photofabrication having an exposure development step using a photosensitive material, inkjet printing or the like. In production of a landless or small-land-width hole, it is important to perform alignment in a step such as hole forming, screen printing, an exposure step or inkjet printing. In particular, a landless or small-land-width hole required for a high-density circuit board needs extremely high alignment accuracy. Most desirably, a land has a uniform width in all directions of a hole, that is, a hole and a land are concentric circles. When alignment is inaccurate, a hole and a land are not concentric circles, disadvantageously.
An example of a method for producing a circuit board by the semi-additive method will be described. First, a through-hole 3 (FIG. 48) is opened on an insulating substrate 1 (FIG. 47), and a thin first conductive layer 12 is provided on the surface including an inner wall of the through-hole (FIG. 49). Then, a plating resist layer 36 is formed in a non-circuit part (FIG. 50). Subsequently, a second conductive layer 13 is formed on the surface of a part on which the first conductive layer 12 is uncovered by electrolytic plating (FIG. 51). Thereafter, the plating resist layer 36 is removed (FIG. 52), and the thin first conductive layer 12 under the plating resist layer 36 is removed by flash etching to form a circuit board (FIG. 53).
A plating resist layer is formed by photofabrication for a high-density circuit board. The photofabrication is generally a method using a negative (photocrosslinkable) or positive (photodegradable) photoresist. In the semi-additive method, since a second conductive layer is provided on the inner wall of a through-hole by electrolytic plating, it is necessary that a plating resist layer does not remain on the through-hole and on the inner wall of the through-hole.
When a negative (photocrosslinkable) dry film photoresist is used, a through-hole and a land are shielded by a shielding part 42 as shown in FIG. 54 to prevent crosslinking of a negative (photocrosslinkable) dry film photoresist 38, and the unreacted dry film photoresist is removed so that the plating resist layer is not present on the through-hole and the land. In these steps, alignment in through-hole forming or an exposure step is important. In particular, a landless or small-land-width through-hole required for a high-density circuit board needs extremely high alignment accuracy.
For example, when the land width is large as shown in FIG. 55(b), even if misalignment occurs by a distance x during alignment of an exposure mask, a through-hole is completely shielded and a negative (photocrosslinkable) dry film photoresist is not crosslinked. However, when the land width is small as shown in FIG. 55(a), if misalignment occurs by the same distance x during alignment of an exposure mask, a through-hole is placed out of a land, making it impossible to form a narrow land on the whole outer periphery of the through-hole, disadvantageously.
For production of a landless through-hole, there is known a method of providing a shielding part 42 as shown in FIG. 56 so that only the center on a through-hole of a negative (photocrosslinkable) dry film photoresist 38 is not exposed, and allowing a plating resist layer 36 to protrude toward the center on the through-hole as shown in FIG. 57, to increase the tolerance of alignment accuracy (e.g. Patent Document 1). When a through-hole 3 has a large diameter as shown in FIG. 58(b), a part of the through-hole 3 is shielded even if a shielding part 42 is misaligned by a distance y. However, when a through-hole has a small diameter as shown in FIG. 58(a), a shielding part 42 is placed out of the through-hole 3 if the shielding part 42 is misaligned by the same distance y. Therefore, a negative (photocrosslinkable) dry film photoresist 38 on the through-hole is crosslinked, and a plating resist layer 36 on the through-hole is not removed, disadvantageously.
Actually, there are limitations to alignment accuracy due to forming hole accuracy, stretching of a substrate, a change in dimension of an exposure photomask, and the like. Through-holes having various diameters are formed on a high-density circuit board, and the number of the holes is also extremely large. Therefore, it is extremely difficult to perform accurate alignment for all through-holes. Accordingly, although a landless or small-land-width through-hole is needed for a high-density circuit board, a small-land-width through-hole must be designed to have a large land width so that the through-hole is surely shielded and a negative (photocrosslinkable) dry film photoresist is not crosslinked, disadvantageously (e.g. Patent Document 2). In a landless through-hole, a shielding part must be designed to be small so that the through-hole is surely shielded and a negative (photocrosslinkable) dry film photoresist is not crosslinked. For this reason, it is difficult for a plating solution to enter the through-hole and plating is not performed, disadvantageously.
A method using an electrodeposition photoresist is also known as a method for forming a plating resist layer. This is a method of forming an electrodeposition photoresist layer uniformly on a conductive layer including the inner wall of a through-hole by electrodeposition coating and then developing the layer by exposure through a photomask to provide a plating resist layer.
An electrodeposition photoresist may be negative (photocrosslinkable) or positive (photodegradable). A positive (photodegradable) electrodeposition photoresist must be degraded by exposure; however, the inside of a columnar through-hole cannot be completely exposed. Therefore, the electrodeposition photoresist in the through-hole cannot be completely degraded and cannot be used as a plating resist layer.
On the other hand, a negative (photocrosslinkable) electrodeposition photoresist is said to be effective as a means for forming a landless through-hole using a photomask having only a landless pattern, since it is not necessary to expose the inside of a through-hole. Since light does not enter a columnar through-hole, it is possible to remove a negative (photocrosslinkable) photoresist layer on the inner wall of the through-hole. However, when the corner of a through-hole has a tapered shape, light partially enters the through-hole, and a plating resist layer on the inner wall of the through-hole cannot be entirely removed, disadvantageously.
When a circuit board is produced by the subtractive method, a conductive layer provided on the inner wall of a hole must be protected by an etching resist layer, so that the conductive layer on the inner wall of the hole is not removed in an etching step. When the etching resist layer is formed using a negative (photocrosslinkable) dry film photoresist, the conductive layer on the inner wall of the hole is protected by tenting, in which a hole and a land are exposed to cover the hole with a crosslinked dry film photoresist, so that an etchant does not enter the hole.
When a hole is protected by tenting, alignment in hole forming or an exposure step is important. In particular, a landless or small-land-width hole required for a high-density circuit board needs extremely high alignment accuracy. Specifically, when the land width is large as shown in FIG. 55(b), a resist cover can be completely formed on a hole even if misalignment occurs by a distance x. However, when the land width is small as shown in FIG. 55(a), if a hole is misaligned from a land by the same distance x, the land is placed out of the hole and an etchant enters the hole, resulting in conduction failure, disadvantageously. However, there are limitations to alignment accuracy due to forming hole accuracy, stretching of a substrate, a change in dimension of an exposure photomask, and the like, actually. Holes having various diameters are formed on a high-density circuit board, and the number of the holes is also extremely large. Therefore, it is extremely difficult to perform accurate alignment for all holes. Accordingly, although a landless or small-land-width hole is needed for a high-density circuit board, the land width must be designed to be large to surely perform tenting, disadvantageously.
Regarding thickness of a dry film photoresist, to perform tenting more surely, the dry film photoresist must be thick for forming a stronger tent. When a surface wiring pattern is formed by etching in the subtractive method, if an etching resist layer formed by a dry film photoresist is thick, an etchant infiltrates poorly during etching and a fine pattern cannot be formed, disadvantageously.
There has been proposed a method for producing a circuit board, which solves the problems of misalignment between a land and a hole caused by alignment and corresponds to a landless or small-land-width hole needed for a high-density circuit board (Patent Document 3). This is a technology of forming a first resin layer on a substrate having a hole, then forming a second resin layer on the surface of the first resin layer other than on the hole, and subsequently dissolving and removing the first resin layer on the hole using a first resin layer developer not dissolving the second resin layer to form an opening on the hole accurately. In this technology, a landless or small-land-width hole can be accurately formed. On the other hand, wet toner or the like is used to form a second resin layer. However, a developing apparatus for toner electrodeposition must be newly introduced for toner electrodeposition using wet toner, because the production cannot be performed only by an existing production apparatus. Accordingly, it is difficult to perform the method when there is no fund or installation space for introduction of new equipment. Even if the equipment can be introduced, management is required for performing stable toner electrodeposition, and there is a risk of a short circuit or disconnected circuit due to abnormal adhesion or insufficient adhesion of toner.
Generally, in a conventional circuit board, the surface of a substrate is covered with a solder resist and a through-hole is filled with the solder resist to prevent soldering of a land, a circuit conductive layer and the like except for a pad necessary for soldering and to maintain insulation properties of the circuit surface and protect the circuit conductive layer. In this case, when the top of a land conductive layer has an acute-angled shape, the top of the solder resist is extremely thin due to surface tension and curing shrinkage of the solder resist, disadvantageously.    Patent Document 1: Japanese Unexamined Patent Publication No. Hei 10-178031    Patent Document 2: Japanese Unexamined Patent Publication No. Hei 07-007265    Patent Document 3: WO 2005/086552